Recently, halogenated polycarbonates have come into extensive use in fields such as flame-retardant molded articles, optical articles, medical instruments, and the like.
Of the above purposes of use, there are fields in which transparency is required. In these fields, foreign materials in molded articles, such as burning, discoloration, and dust particles, which are attributable to metal corrosion, etc., greatly impair the commercial value of the molded articles.
For this reason, many studies have been made to improve the thermal stability of raw material polycarbonate resins and for other purposes. However, discoloration and contamination caused by materials of the injection molding machines during injection molding have received no attention, and burning, discoloration, contaminants, and other foreign materials in molded articles have been regarded as being attributable to the heat resistance and other properties of the raw material polycarbonates.
It has therefore been thought that in order to eliminate the defects in molded articles such as burning, discoloration, and inclusion of contaminants, raw resin manufacturers should improve the thermal stability of raw polycarbonates.
Injection molding machines have been suitably selected by raw resin manufacturers or users of the raw resins from various kinds of injection molding machines supplied by manufacturers thereof, and the raw resin manufacturers and the resin users have been unable to afford to consider details of their injection molding machines, or there has conventionally been no necessity of doing so. On the other hand, ordinary injection molding machine manufacturers also have had no knowledge as to the fact that the degree of discoloration of molded articles varies depending on materials of the injection molding machine used. Further, it is difficult to say that the meaning of the recent molding processes in which those molded articles have come to be produced under exceedingly severe conditions as compared to conventional processes has been properly understood.
In an attempt to overcome the above-described problems, the present inventors conducted extensive studies. As a result, it was found that when a molten halogenated polycarbonate resin is contacted with a low-alloy steel such as SACM steel (Al-Cr-Mo steel), SCM steel (Cr-Mo steel), or the like, which is one kind of ordinary carbon steel, the surface of the steel is corroded severely and the resin is decomposed to turn yellow and, in extreme cases, the resin further turns black. It was also found that when a halogenated polycarbonate resin or a composition thereof is molded after being retanied at 320.degree. C. in the cylinder of an injection molding machine, which is made of a nitrided steel obtained from SACM steel, SCM steel, or the like, the resin or composition thereof discolors to give black molded articles.
From the above, it is apparent that most of the defects, such as burning, discoloration, and contaminants, in halogenated polycarbonate molded articles have been newly formed in the injection molding machine during the re-melting and plasticization of the solid polycarbonate and injection of the melt. It has therefore been presumed that in order to produce halogenated polycarbonate molded articles, it is essential to improve the molding machine, specifically to replace materials thereof with new kinds of materials which undergo little metal corrosion and cause little burning, discoloration, and formation of undesirable substances. However, a material has not yet been found which is usable for manufacturing the cylinder, screw, or other parts of an injection molding machine, undergoes only little metal corrosion by halogenated polycarbonate resins or compositions thereof, and causes only little burning, discoloration, and formation of undesirable substances, and which can be processed economically.